This invention relates to a rotational speed control circuit for a D.C motor and particularly pertains to a control circuit for a variable rotational speed motor for use in a blower of an air conditioner for a motor vehicle.
In a conventional control circuit for a D.C motor, there is provided a power transistor with its emitter-collector path connected in series with the D.C motor and a D.C power source. The speed control of the D.C motor is accomplished by changing a base current delivered to the power transistor thereby varying a current flow passing through the D.C motor. In case where the D.C motor is used for a blower of an air conditioner for motor vehicles, an undersirable change in the rotational speed of the motor is caused by variations in a load and a source voltage of the motor due to the changing-over of the air passage and other operational conditions. The undesirable change in the rotational speed will invite the shortage of the amount of air blown and provide an enhancement of a blower noise.
It is accordingly desired for the blower to operate at substantially constant speed without regard to changes in a load and a source voltage. The rotational speed of the blower is also desired to be controllable over a wide range in a continuous manner so that the amount of air blown can be continuously adjusted in response to various requirements.
In order to meet the desirabilities mentioned above, it is well known to provide a differential amplifier for the detection of a change in the voltage across the D.C motor. The output of the differential amplifier is delivered to a base electrode of a power transistor with its emitter-collector path connected in series with the motor so that the voltage across the motor may be maintained at substantially constant value. The rotational speed of the motor is adjustable by changing a resistance of an element connected to a common emitter of the differential amplifier.
A disadvantage of the above-mentioned prior art is that a controllable range of the rotational speed of the D.C motor is insufficient for use in the blower of the air conditioner. This is because a base current of the power transistor can be controlled only within the range of collector current flowing through one of the differential amplifier transistors to which the power transistor is connected. The current flow passing through the impedance element connected to the common emitter of the differential amplifier is divided into two to flow through each emitter-collector path of transistors. It follows therefore that the output of the differential amplifier, that is, the collector current of one of the transistors becomes relatively small in value.